1. Field of the Invention
The present invention is directed to radical-curable adhesive compositions which include a (meth)acrylate component; a thermal resistance-conferring component; and a radical cure-inducing composition. Reaction products of the compositions of this invention exhibit superior resistance to thermal degradation.
2. Brief Description of Related Technology
Radical-curable adhesive compositions generally are well-known. In the context of anaerobic adhesives, see e.g., R. D. Rich, xe2x80x9cAnaerobic Adhesivesxe2x80x9d in Handbook of Adhesive Technology, 29, 467-79, A. Pizzi and K. L. Mittal, eds., Marcel Dekker, Inc., New York (1994) and references cited therein. In the context of radiation-curable adhesives, see e.g., J. G. Woods, xe2x80x9cRadiation Curable Adhesivesxe2x80x9d in Radiation Curing: Science and Technology, 333-98, S. P. Pappas, ed., Plenum Press, New York (1992).
Uses of radical-curable adhesives are legion and new applications continue to be developed.
In the past, many adhesives particularly anaerobic adhesives, have been rendered resistant to degradation at elevated temperatures by the inclusion of certain additives.
For instance, U.S. Pat. No. 3,988,299 (Malofsky) refers to a heat curable composition having improved thermal properties, which includes certain acrylate monomers and maleimide compounds.
L. J. Baccei and B. M. Malofsky, xe2x80x9cAnaerobic Adhesives Containing Maleimides Having Improved Thermal Resistancexe2x80x9d in Adhesive Chemicals, 589-601, L-H, Lee, ed., Plenum Publishing Corp. (1984) report the use of maleimidesxe2x80x94specifically, N-phenyl maleimide, m-phenylene dimaleimide and a reaction product of methylene dianiline and methylene dianiline bismaleimidexe2x80x94to increase the thermal resistance of anaerobic adhesives which are fully cured at temperatures of at least 150xc2x0 C.
U.S. Pat. No. 4,216,134 (Brenner) speaks to one-component anaerobic adhesive compositions which include ethylenically unsaturated diluent monomers (such as styrene, divinylbenzene, diallyl carbonates, diallyl maleate, diallyl phthalate, diallyl isophthalate and the like), prepolymers (such as 1,2-polybutadienes and copolymers thereof, isophthalic polymers, bisphenol A fumates, epoxy resins, polyallylvinyl ethers and the like) and triallyl cyanurate or triallyl isocyanurate as reaction components. The ""134 patent is at least a three component composition (in addition to its cure components) and requires a prepolymer which may be included in addition to or as a replacement for the ethylenically unsaturated diluent monomer. Examples of the prepolymer are given as xe2x80x9chigh vinyl 1,2-polybutadienes and copolymers thereof especially styrene; isophthalic polymers; bisphenol A fumates and other alkyls; epoxy resins; polyalkyl vinylethers and related polymers; alkylic resins based on polyfunctional ethers and esters and mixtures of two or more of these prepolymers.xe2x80x9d (Col. 5, lines 32-39.)
While the addition to radical-curable adhesive compositions of such maleimide compounds to render them resistant to thermal degradation provides reaction products with acceptable performance, it would be desirable to find alternative compounds to include in such formulations. Moreover, in certain adhesive compositions, maleimides (which tend to be, insoluble) often function to reduce radiation penetration thereby inhibiting photoinitated cure of acrylate-based compositions.
U.S. Pat. No. 4,540,829 (Heffner) speaks to alkylated di and polycyclopentadiene diphenols.
U.S. Pat. No. 5,495,051 (Wang) speaks to certain phenol alkyl ethers, their preparation and cure together with bismaleimides.
U.S. Pat. No. 5,166,290 (Hayashi) describes a resin composition for composites that requires a bismaleimide mixture with certain allyl phenyl ethers.
U.S. Pat. No. 5,084,490 (McArdle) and U.S. Pat. No. 5,141,970 (McArdle) describe polyfunctional cationically polymerizable styryloxy compounds, curable compositions including such styryloxy compounds and methods of forming high-temperature resistant polymers therefrom. These compounds may be described in more detail with reference to structure I: 
where R1 and R2 are H, or one of R1 and R2 is H and the other is alkyl; R3 and R4 (which may be the same or different) are H, C1-5 alkyl or C1-5 alkenyl; or one of R3 and R4 may be xe2x80x94OR5 or C1-5 alkoxy or C1-5 alkenyloxy, if R2 is not methyl; and R5 is selected from 
where R6 is C1-5 alkyl, and R7, R8 and R9 may be the same or different and are H or C1-5 alkyl.
These cationically curable propenyloxystyrene compounds possess outstanding thermal and mechanical properties in their cured state. To reach the cured state, the compounds undergo a 2-stage curing process involving an initial acid catalyzed addition polymerization or copolymerization of the styrene group (called A-stage polymerization), followed by a heat-triggered, post-curing reaction of the propenyloxyphenyl group (called B-stage polymerization). During the post-curing reaction, the A-stage polymer rearranges to form a reactive phenolic polymer, which spontaneously reacts with the propenyloxy group through an electrophilic substitution reaction. This results in the formation of a cross-linked polymer that exhibits a high decomposition temperature (xe2x80x9cTdxe2x80x9d) [Td greater than 400xc2x0 C., as measured by thermal gravimetric analysis (xe2x80x9cTGAxe2x80x9d)], a high glass transition (xe2x80x9cTgxe2x80x9d) [Tg greater than 300xc2x0 C., as measured by dynamic mechanical analysis (xe2x80x9cDMAxe2x80x9d)] and good adhesion. See also J. Woods et al., xe2x80x9cAlkenyloxy Styrene Monomers for High-Temperature Adhesives and Sealantsxe2x80x9d in Photopolymerization, ch. 9, pp. 107-20 (1997).
While this information describes styryloxy and propenyloxystyrene compounds and suggests their usefulness as adhesives, sealants and/or coatings and/or in compositions destined for such use, their use to confer resistance to thermal degradation on such compositions, particularly (meth)acrylate-containing radical-curable adhesive compositions, remained unknown until the discovery of the invention described herein.
U.S. Pat. No. 5,369,200 (Schadeli) describes terpolymers of two different maleimide monomers and an olefinically unsaturated phenyl ether monomer in which the phenyl ether is defined by an acid cleavable group xe2x80x94OR2 linked to a phenyl ring. Examples of such monomers are given as a terpolymer of 4-(2-tetrahydropyranyloxy) benzyl methacrylate, N-hydroxymethylmaleimide and N-(acetoxymethyl)maleimide. The terpolymers are used as positive resists with acid generating photocatalysts and therefore it is imperative that the OR2 group be selected to be acid cleavable.
U.S. Pat. No. 4,387,204 (Zahir) and U.S. Pat. No. 4,468,524 (Zahir) describe alkenylphenyl substituted acrylates or methacrylates and cross-linkable compositions thereof.
Notwithstanding the state-of-the-technology, there is an on-going search for additives to improve the thermal performance of reaction products of radical-curable adhesives. In addition, it would be desirable to provide alternatives, replacements and/or supplements for maleimide-type materials for improving the resistance to thermal degradation of reaction products of radical-curable adhesive compositions.
In particular, it would be desirable to provide additives that tend to be soluble which would lead to adhesive compositions having improved homogenity.
The present invention meets the desire discussed above by providing radical-curable adhesive compositions, reaction products of which exhibit superior performance at elevated temperatures. The compositions include a (meth)acrylate component; a thermal resistance-conferring component; and a radical cure-inducing composition.
Within the thermal resistance-conferring component are polymerizable aromatic materials having at least one Claisen rearrangable functional group. Such Claisen rearrangable functional groups include allyloxy and allylthiol groups, where the oxygen and the sulfur, respectively, are bound to the aromatic ring. Desirably, such polymerizable aromatic materials should include at least two reactive functional groups, at least one of which is a Claisen rearrangable functional group. In the situation where a reactive functional group other than a Claisen rearrangable one is also present on thermal resistance-conferring component, such a group may be selected from (meth)acrylate, vinyl, styryloxyl, and combinations thereof.
Thermal resistance-conferring components capable of undergoing Claisen rearrangement include those aromatic materials within structure II below: 
where
Ar is an aromatic ring or ring system substituted with a Claisen rearrangable functional group, having at least one aromatic ring which may be joined or fused to additional rings of an aromatic or non-aromatic nature;
A is O or S;
R10, R11 and R12 may be the same or different and are selected from H, alkyl (such as C1-3), phenyl or substituted derivatives;
R13 and R14 may be the same or different and are selected from H, or Axe2x80x94CH2xe2x80x94CR12xe2x95x90CR11R12, where A, R10, R11 and R12 are as defined above;
n is 1 or 2, provided that
when n is 1, Z and Zxe2x80x2 are not present when Y is H, Z or Z1 are present when Y is carbonyl, sulfoxide, or sulfone, and Z and Z1 are present when Y is linear, branched, cyclic or polycyclic alkyl or alkenyl (such as C1-60), or bisalkyl phenylene, and Z or Z1 may be the same or different and are selected from H, linear or branched alkyl, hydroxy alkyl or carboxy alkyl (such as C1-6); and
when n is 2, Z or Z1 is present when Y is alkyl or alkenyl, and Z or Z1 may be the same or different and are selected from H, linear or branched alkyl, hydroxy alkyl or carboxy alkyl (such as C1-6), and Z or Z1 is not present when Y is carbonyl, sulfoxide, sulfone, heteroatoms (such as O or S) or a single bond.
The invention also provides a process for preparing reaction products from the radical-curable adhesive compositions of the present invention, the steps of which include applying the composition to a desired substrate surface and exposing the coated substrate surface to conditions which are appropriate to effect cure thereofxe2x80x94e.g., exposure to conditions in which air is substantially excluded therefrom for anaerobic applications, exposure to radiation in the electromagnetic spectrum for photoinitated applications, oxygen in the atmosphere for oxygen triggered cure or heating for thermally activated cure.
Also, the invention provides the reaction products so-formed by the above-described process, which reaction products demonstrate superior thermal properties, such as resistance to degradation at elevated temperatures.
The present invention will be more fully appreciated by a reading of the section entitled, xe2x80x9cDetailed Description of the Inventionxe2x80x9d, together with the illustrative examples which follow thereafter and the figures.